Why is the velocity of flow of water higher through the smaller hole than through the larger hole?
This can be explained by the law of continuity. Let V1 be the velocity of the water flow through the hole (assuming circular) of area S1 and V2 be the velocity of water flow through the hole of area S2. By the law of continuity V1*S1 = V2*S2 that is if the S1>S2, V2>V1. It can also by explained in terms of flow rate. No matter what the size of hole is, the flow rate would always be the same, it can either be compensated by the area of the hole or by the velocity of the water flow.
Why does the starch when reacted with iodine forms turns into blue color?
Remember that iodine when reacts with starch does not form a bond with it. Starch is kind of helical and the iodine is inserted into that helix. The electrons of iodine interacts with electrons in starch resulting in the formation of blue colour.
Why are the Carbanions unstable when they are bonded with Electron donating group?
A carbanion contains negative charge ie it contains an extra valency which wants to get rid of. If it is bonded with an atom that donates electron, it is further destabilized. Remember, a carbanion always seeks to stabilize itself, be it my means of resonance (distributing electron all around) or be it through bonding with other carbocation (which has deficiency of an electron).